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Monday, April 2, 2012

Managing Tablespaces and Data files


Managing Tablespaces and Data files

A small database might need only the SYSTEM tablespace; however, Oracle recommends that you create additional tablespaces to store user data, user indexes, undo segments, and temporary segments separate from data dictionary. This gives you more flexibility in various database administration operations and reduces contention among dictionary objects and schema objects for the same data files.

The DBA can create new tablespaces, resize data files, add data files to tablespaces, set and alter default segment storage settings for segments created in a tablespace, make a tablespace read-only or read-write, make a tablespace temporary or permanent, and drop tablespaces.

Creating New Tablespaces
You can create Locally Managed or Dictionary Managed Tablespaces. In prior versions of Oracle only Dictionary managed Tablespaces were available but from Oracle ver. 8i you can also create Locally managed tablespaces. The advantages of locally managed tablespaces are
Locally managed tablespaces track all extent information in the tablespace itself by using bitmaps, resulting in the following benefits:
  • Concurrency and speed of space operations is improved, because space allocations and deallocations modify locally managed resources (bitmaps stored in header files) rather than requiring centrally managed resources such as enqueues
  • Performance is improved, because recursive operations that are sometimes required during dictionary-managed space allocation are eliminated
To create a locally managed tablespace give the following command
SQL> CREATE TABLESPACE usertbs DATAFILE '/u02/oracle/ica/ica01.dbf' SIZE 50M
    EXTENT MANAGEMENT LOCAL AUTOALLOCATE;
AUTOALLOCATE causes the tablespace to be system managed with a minimum extent size of 64K.
SQL> CREATE TABLESPACE usertbs DATAFILE '/u02/oracle/ica/ica01.dbf'                       SIZE 50M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 256K;
To Create Dictionary Managed Tablespace
SQL> CREATE TABLESPACE usertbs DATAFILE '/u02/oracle/ica/ica01.dbf' SIZE 50M EXTENT MANAGEMENT DICTIONARY;

Bigfile Tablespaces (Introduced in Oracle Ver. 10g)

A bigfile tablespace is a tablespace with a single, but very large (up to 4G blocks) datafile. Traditional smallfile tablespaces, in contrast, can contain multiple datafiles, but the files cannot be as large. Bigfile tablespaces can reduce the number of datafiles needed for a database.
To create a bigfile tablespace give the following command
SQL> CREATE BIGFILE TABLESPACE ica_bigtbs
    DATAFILE '/u02/oracle/ica/bigtbs01.dbf' SIZE 50G;

To Extend the Size of a tablespace

Option 1

You can extend the size of a tablespace by increasing the size of an existing datafile by typing the following command

SQL> alter database usertbs datafile ‘/u01/oracle/data/icatbs01.dbf’ resize 100M;

This will increase the size from 50M to 100M

Option 2

You can also extend the size of a tablespace by adding a new datafile to a tablespace. This is useful if the size of existing datafile is reached o/s file size limit or the drive where the file is existing does not have free space. To add a new datafile to an existing tablespace give the following command.

SQL> alter tablespace add datafile     ‘/u02/oracle/ica/icatbs02.dbf’ size 50M;

Option 3

You can also use auto extend feature of datafile. In this, Oracle will automatically increase the size of a datafile whenever space is required. You can specify by how much size the file should increase and Maximum size to which it should extend.
To make a existing datafile auto extendable give the following command
SQL> alter database datafile ‘/u01/oracle/ica/icatbs01.dbf’ auto extend ON next 5M maxsize 500M;

 

To decrease the size of a tablespace

You can decrease the size of tablespace by decreasing the datafile associated with it. You decrease a datafile only up to size of empty space in it. To decrease the size of a datafile give the following command
SQL> alter database datafile ‘/u01/oracle/ica/icatbs01.dbf’      resize 30M;

Taking tablespaces Offline or Online

You can take an online tablespace offline so that it is temporarily unavailable for general use. The rest of the database remains open and available for users to access data. Conversely, you can bring an offline tablespace online to make the schema objects within the tablespace available to database users. The database must be open to alter the availability of a tablespace.
To alter the availability of a tablespace, use the ALTER TABLESPACE statement. You must have the ALTER TABLESPACE or MANAGE TABLESPACE system privilege.
To Take a Tablespace Offline give the following command
SQL>alter tablespace usertbs offline;
To again bring it back online give the following command.
SQL>alter tablespace usertbs online;
To take individual datafile offline type the following command
SQL>alter database datafile ‘/u01/oracle/ica/ica_tbs01.dbf’ offline;
Again to bring it back online give the following command
SQL> alter database datafile ‘/u01/oracle/ica/ica_tbs01.dbf’ online;

Making a Tablespace Read only

Making a tablespace read-only prevents write operations on the datafiles in the tablespace. The primary purpose of read-only tablespaces is to eliminate the need to perform backup and recovery of large, static portions of a database. Read-only tablespaces also provide a way to protecting historical data so that users cannot modify it. Making a tablespace read-only prevents updates on all tables in the tablespace, regardless of a user's update privilege level.
To make a tablespace read only
SQL>alter tablespace ica read only
Again to make it read write
SQL>alter tablespace ica read write;

Renaming Tablespaces

Using the RENAME TO clause of the ALTER TABLESPACE, you can rename a permanent or temporary tablespace. For example, the following statement renames the users’ tablespace:
Sql>ALTER TABLESPACE users RENAME TO usersts;

Dropping Tablespaces

You can drop a tablespace and its contents (the segments contained in the tablespace) from the database if the tablespace and its contents are no longer required. You must have the DROP TABLESPACE system privilege to drop a tablespace.

Caution: Once a tablespace has been dropped, the data in the tablespace is not recoverable. Therefore, make sure that all data contained in a tablespace to be dropped will not be required in the future. Also immediately before and after dropping a tablespace from a database, back up the database completely.
To drop a tablespace give the following command.
SQL> drop tablespace usertbs;
This will drop the tablespace only if it is empty. If it is not empty and if you want to drop it anyhow then add the following keyword
SQL>drop tablespace usertbs including contents;
This will drop the tablespace even if it is not empty. But the datafiles will not be deleted you have to use operating system command to delete the files.
But If  you include datafiles keyword then, the associated datafiles will also be deleted from the disk.
SQL>drop tablespace usertbs including contents and datafiles;

Temporary Tablespace

Temporary tablespace is used for sorting large tables. Every database should have one temporary tablespace. To create temporary tablespace give the following command.

SQL>create temporary tablespace temp tempfile ‘/u01/oracle/data/ica_temp.dbf’ size 100M extent management local uniform size 5M;

Increasing or Decreasing the size of a Temporary Tablespace

You can use the resize clause to increase or decrease the size of a temporary tablespace. The following statement resizes a temporary file:

SQL>ALTER DATABASE TEMPFILE '/u02/oracle/data/lmtemp02.dbf' RESIZE 18M;

The following statement drops a temporary file and deletes the operating system file:
SQL> ALTER DATABASE TEMPFILE '/u02/oracle/data/lmtemp02.dbf' DROP INCLUDING DATAFILES;
Default Temporary Tablespace
After database creation, a default temporary tablespace can be set by creating a temporary
tablespace and then altering the database.
Sql>ALTER DATABASE DEFAULT TEMPORARY TABLESPACE temp;

Viewing Information about Tablespaces and Data files

Oracle has provided many Data dictionaries to view information about tablespaces and datafiles.  Some of them are:
To view information about Tablespaces in a database give the following query
SQL>select * from dba_tablespaces
SQL>select * from v$tablespace;
To view information about Datafiles
SQL>select * from dba_data_files;
SQL>select * from v$datafile;
To view information about Tempfiles
SQL>select * from dba_temp_files;
SQL>select * from v$tempfile;
To view information about free space in datafiles
SQL>select * from dba_free_space;
To view information about free space in tempfiles
SQL>select * from V$TEMP_SPACE_HEADER;

Relocating or Renaming Datafiles

You can rename datafiles to either change their names or relocate them.

Renaming or Relocating Datafiles belonging to a Single Tablespace

To rename or relocate datafiles belonging to a Single Tablespace do the following.
1. Take the tablespace offline
2.  Rename or Relocate the datafiles using operating system command
3.  Give the ALTER TABLESPACE with RENAME DATAFILE option to change the filenames within the Database.
4.   Bring the tablespace Online
For Example suppose you have a tablespace users with the following datafiles
        /u01/oracle/ica/usr01.dbf’
        /u01/oracle/ica/usr02.dbf’
Now you want to relocate /u01/oracle/ica/usr01.dbf’  to ‘/u02/oracle/ica/usr01.dbf’ and want to rename ‘/u01/oracle/ica/usr02.dbf’ to ‘/u01/oracle/ica/users02.dbf’  then follow the given the steps
1.       Bring the tablespace offline
SQL> alter tablespace users offline;
2.       Copy the file to new location using o/s command.
$cp /u01/oracle/ica/usr01.dbf  /u02/oracle/ica/usr01.dbf’
Rename the file ‘/u01/oracle/ica/usr02.dbf’ to ‘/u01/oracle/ica/users02.dbf’ using o/s command.
$mv  /u01/oracle/ica/usr02.dbf /u01/oracle/ica/users02.dbf
3.       Now start SQLPLUS and type the following command to rename and relocate these files
SQL> alter tablespace users rename file
   ‘/u01/oracle/ica/usr01.dbf’, ‘/u01/oracle/ica/usr02.dbf’ to                    
   ‘/u02/oracle/ica/usr01.dbf’,’/u01/oracle/ica/users02.dbf’;
4.       Now bring the tablespace  Online
SQL> alter tablespace users online;

Procedure for Renaming and Relocating Datafiles in Multiple Tablespaces

You can rename and relocate datafiles in one or more tablespaces using the ALTER DATABASE RENAME FILE statement. This method is the only choice if you want to rename or relocate datafiles of several tablespaces in one operation. You must have the ALTER DATABASE system privilege
To rename datafiles in multiple tablespaces, follow these steps.
1. Ensure that the database is mounted but closed.
2.  Copy the datafiles to be renamed to their new locations and new names, using the operating system..
3.  Use ALTER DATABASE to rename the file pointers in the database control file.
For example, the following statement renames the datafiles/u02/oracle/rbdb1/sort01.dbf and /u02/oracle/rbdb1/user3.dbf to /u02/oracle/rbdb1/temp01.dbf and /u02/oracle/rbdb1/users03.dbf, respectively:
ALTER DATABASE
    RENAME FILE '/u02/oracle/rbdb1/sort01.dbf',
                '/u02/oracle/rbdb1/user3.dbf'
             TO '/u02/oracle/rbdb1/temp01.dbf',
                '/u02/oracle/rbdb1/users03.dbf;
 Always provide complete filenames (including their paths) to properly identify the old and new datafiles. In particular, specify the old datafile names exactly as they appear in the DBA_DATA_FILES view.
 
4. Back up the database. After making any structural changes to a database, always perform an immediate and complete backup.
5.      Start the Database
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